The present invention relates to a power supply circuit, and more specifically relates to a self-adjusting power supply circuit of silicon controlled dimming in LED lighting.
Silicon controlled dimming is a lighting adjustment method commonly applied to incandescent lights and energy-saving lights. In recent years, due to the developments of LED lighting technology, silicon controlled dimming technology is also applied to LED lights, and now widely applied to technical fields such as household and commercial lightings. FIG. 2 schematically illustrates a circuit diagram of silicon controlled dimming in LED lighting. V0 represents AC mains voltage, which is generally 220 VAC or 120 VAC; reference number 310 represents a silicon controlled dimmer; reference 311 represents a rectifier bridge; reference number 312 represents buck switching power supply; and reference 313 represents a load which is an LED light. When the complete AC mains voltage is chopped by the silicon controlled dimmer, a broken AC waveform is formed; after rectification, the broken waveform supplies power to the buck switching power supply 312. The more broken the waveform is, the dimmer the LED light becomes.
FIGS. 3a to 3c disclose waveforms of the voltage at two ends of a capacitor CHV after rectification. An initial wave form 110 represents a complete AC mains voltage waveform, which is a waveform of a sinusoidal waveform after rectification, and this waveform corresponds to the situation where the dimmer is not connected. A third waveform 113 represents a chopped wave chopped by the silicon controlled dimmer 310 and an output waveform rectified by the rectifier bridge 311. An area enclosed by a first waveform 111, a second waveform 112 and the time axis represents the chopped parts chopped by the dimmer. The larger this area is (the more the second waveform 112 shifts to the right), the larger the parts having zero electrical level are, and correspondingly, the dimmer the LED light will become. A fourth waveform 114 represents a waveform with the largest parts being chopped off when the dimming effect of the dimmer is set at the greatest degree. According to the fourth waveform 114, most parts have zero electrical level, and only a very small portion of the waveform is left with non-zero electrical level. In such situation, power supply becomes a very difficult condition to maintain when electrical level is zero for most of the time. If power supply is insufficient, the drive chip will restart and the LED light will flash as a result. However, flashing of the LED light is not preferred.
As noted above, power supply is a very important technical issue in a silicon controlled dimming circuit. Some of the existing power supply technologies are described as follows. In a first existing power supply technology as shown in FIG. 4, Vi represents a voltage of a first end capacitor CHV1; Vi is connected with a VCC terminal via an eleventh resistor R11 to directly supply power to the drive circuit. The greatest disadvantage of this technology is that the eleventh resistor R11 has to allow current of small enough voltage to pass through so as to allow power supply even under the voltage represented by the fourth waveform 114 shown in FIG. 3c, but when the waveform is near to its full cycle, power consumption will be enormously high (wasted through direct grounding of a first clamp tube Z1), and there is a great loss of efficiency. This technology is therefore rarely used since people nowadays attach more and more importance to efficiency. This first existing power supply technology uses components such as a tenth resistor R10, the eleventh resistor R11, a twelfth resistor R12, the first clamp tube Z1, a fifth capacitor C5, a third diode D3 and a sixth capacitor C6.
FIG. 5 shows a second existing power supply technology according to which a junction gate field-effect transistor (JFET) J10 is integrated inside the drive chip to directly supply power to the VCC terminal. This technology effectively solves the dilemma between power supply and efficiency. However, its greatest disadvantage is that the drive chip has to be designed in accordance with high-voltage BCD. High-voltage BCD has a much higher cost and the space required by each component is larger. Another deadly disadvantage is that, under high-voltage BCD, ESD of the JFET is not good, and thus resulting in easy damage of chip pins and lowered overall product yield rate along with complaints from the customers. This second existing power supply technology uses components such as the JFET J10, a sixteenth resistor R16, a seventeenth resistor R17, an eighteenth resistor R18, an eighth capacitor C8, a ninth capacitor C9 and a fifth diode D5.
FIG. 6 shows a third existing power supply technology according to which the drive chip is designed in accordance with ordinary prior arts, and a power supply network comprising a tenth secondary winding L10, an eleventh secondary winding L11, a twelfth diode D12, a fourth capacitor C4 and a ninth resistor R9 is formed to supply power to the VCC terminal. This technology solves the problems of the first two technologies described above, and therefore being widely used nowadays. This technology also uses a second clamp diode Z2, a seventh resistor R7, a third capacitor C3, a sixth resistor R6 and a second capacitor C2.
FIG. 7 shows a fourth existing power supply technology to be used in a switching power supply having a floating ground structure (a variation of the third existing power supply technology), where one less winding is being used. In other words, a power supply network formed by a fourteenth diode D14, a seventh capacitor C7 and a fifteenth resistor R15 supplies power to the VCC terminal. This technology also uses a tenth resistor R10, an initial capacitor C0, a first capacitor C1, a thirteenth resistor R13, a twelfth diode D10 and a third clamp diode Z3.
The third and the fourth existing power supply technologies are widely used power supply technologies nowadays. The third one having a common ground structure (FIG. 6) has disadvantages of requiring additional windings and a network formed by high-voltage diode, high-voltage capacitors and resistors for power supply. The fourth one having a floating ground structure (FIG. 7) does not require any secondary windings, but it still requires components such as high-voltage diodes, high-voltage capacitors and resistors. The large quantity of components increases the cost and complicates the wire arrangement on a PCB. In actual use, it is discovered that in order to adapt to different dimmers, RC network has to be more complicated to solve the problem of power supply. Besides, as shown in FIGS. 6-7, the second clamp diode Z2 and the third clamp diode Z3 are used for consuming the extra power at the VCC terminal, thereby increasing power consumption and lowering efficiency.